The present invention relates generally to the field of heat treating metal castings and the field of reclaiming sand from sand cores and sand molds used to make metal castings.
Generally, prior art methods and apparatus require that two or three distinctly separate steps be taken in order to heat treat a metal casting formed by a permanent mold or sand mold with a sand core, and reclaim sufficiently pure sand from the sand mold or sand core. The present invention allows for heat treating and reclamation of sufficiently pure sand in a single step.
Methods and apparatus for manufacturing metal castings are well known. Molds and cores are used to displace molten material so that when the molten material is solidified, a casting is formed that reflects the features of the mold and core. Molds have the exterior features of the casting formed on the interior walls of the mold and cores have the interior features of the casting formed on the exterior surface of the core. The cores are typically made from sand whereas the molds are sometimes made from sand. Sand molds and cores are typically pre-molded from a mixture of sand and a combustible binder. For simplicity, sand molds and sand cores are referred to hereafter as simply sand cores.
In accordance with some of the prior art, once the casting is formed, three distinctly different steps are carried out in order to heat treat the metal casting and reclaim sufficiently pure sand from the sand core. The first step separates portions of sand core from the casting. The sand core is typically separated from the casting by one or a combination of means. For example, sand may be chiseled away from the casting or the casting may be physically shaken to break-up the sand core and remove the sand. Once the sand is removed from the casting, the second and third steps are carried out. In this typical, three-step prior art, the order in which the second and third steps are taken is not important, since the sand has already been separated from the casting. The second step consists of heat treating the casting. The casting is typically heat treated if it is desirable to strengthen or harden the casting. The third step consists of purifying the sand that was separated from the casting. The purification processes is typically carried out by one or a combination of means. These may include burning the binder that coats the sand, abrading the sand, and passing portions of the sand through screens. It is important that the reclaimed sand be sufficiently pure in order for it to be properly reused in the construction of new sand cores. It is also helpful if the reclaimed sand is rounded, at least to some degree, so as to assist in the casting of smooth surfaces and to assist in good bonding of the sand grains which causes strong cores. Therefore, portions of sand may be re-subjected to reclaiming processes until sufficiently pure sand is reclaimed.
The purity of the reclaimed sand can be measured in terms of the quantity of unburned binder. The less unburned binder, the more pure the sand. While seeking increased purity, some sand is reduced to "fines". Fines is the term used for sand particles smaller than a specified size. Fines are so small that they require excessive amounts of binder. These two measures (purity and fines) generally oppose each other in that the higher the measure of one, the lower the measure of the other. It is important to balance these measures; therefore, it is important that the sand reclaiming processes be capable of controlling these measures.
In accordance with the present inventor's previous invention disclosure of U.S. Pat. No. 5,294,094, only one step need be taken in order to heat treat metal castings formed by sand cores and reclaim sand from the sand cores. This is carried out by introducing the castings, with the sand cores attached thereto, into a furnace with an oxygenated atmosphere that is heated to at least the combustion temperature of the sand core binder material. This causes combustion of some of the binder of the sand core which, in combination with other means, causes the sand core to separate from the casting. The system disclosed in U.S. Pat. No. 5,294,094 promotes more binder combustion than is required to separate the sand core from the casting. The system disclosed in U.S. Pat. No. 5,294,094 ejects sand from the furnace in a sufficiently pure state for some applications; but, that system is not capable of combusting a sufficient amount of binder (or otherwise processing the sand core) so as to render sand that is sufficiently pure for certain other applications. Also, that system does not make provisions for varying the characteristics of the reclaimed sand; no selective control over sand roundness, amount of fines, or amount of unburned binder in the reclaimed sand is possible. Therefore, the sand reclaimed using the method and apparatus disclosed in U.S. Pat. No. 5,294,094 may require further processing in order to obtain sand that is sufficiently pure for certain applications or sand that has certain characteristics. Therefore, previous sand reclaiming systems are inherently inefficient in that they require at least a two step process, carried out in two separate venues by separate, specialized equipment, in order to heat treat a metal casting formed by a sand core and reclaim sufficiently pure sand from the sand core.
There is a need, therefore, for a more efficient method, and associated apparatus, that allows for more efficient heat treatment, sand core removal, and reclamation of sufficiently pure sand from the sand core.